The development of display devices using light emitting elements such as electroluminescence (EL) elements and the like has become active in recent years. The light emitting element is a self-emitting element, so that the light emitting display device has high visibility, and is suited for being thinner because a back light which is necessary for liquid crystal display devices (LCDs) and the like is not needed, and that there are almost no limitations on their angle of view.
The term EL element indicates an element having a light emitting layer in which luminescence generated by the application of an electric field can be obtained. There are light emission when returning to a base state from a singlet excitation state (fluorescence), and light emission when returning to a base state from a triplet excitation state (phosphorescence) in the light emitting layer, and a light emitting device of the present invention may use either of the aforementioned types of light emission.
EL elements normally have a laminate structure in which a light emitting layer is sandwiched between a pair of electrodes (anode and cathode). A laminate structure having “an anode, a hole transporting layer, a light emitting layer, an electron transporting layer, and a cathode”, proposed by Tang et al. of Eastman Kodak Company, can be given as a typical structure. This structure has extremely high efficiency light emission, and most of the EL elements currently being researched employ this structure.
Further, structures having the following layers laminated in order between an anode and a cathode also exist: a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer; and a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer. Any of the above-stated structures may be employed as EL element structure used in the light emitting device of the present invention. Furthermore, fluorescent pigments and the like may also be doped into the light emitting layer.
All layers formed in EL elements between the anode and the cathode are referred to generically as “EL layers” in this specification. The aforementioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer are all included in the category of EL layers, and light emitting elements structured by an anode, an EL layer, and a cathode are referred to as electro luminescence elements.